1. Field of the Invention
The present invention relates to an ink-jet head which selectively deposits ink droplets on an image recording medium, and the methods of manufacturing and driving the same.
2. Description of the Related Art
Of non-impact printers which are largely increasing their share in the market nowadays, ink-jet printers are simplest in principle, and also suitable for color printing. Of the ink-jet printers, so-called drop-on-demand (DOD) type ink-jet printers are the most popular, which eject ink droplets only at the time of forming dots.
As representative head systems in the DOD type ink-jet printers, for example, there is a Kaiser type one as disclosed in Japanese Patent Publication No. 53-12138 or a thermal jet type one as disclosed in Japanese Patent Publication No. 61-59914.
However, they have troublesome problems that the Kaiser type ink-jet head described in the Japanese Patent Publication No. 53-12138 is hard to be small-sized, while the thermal-jet type ink-jet head described in the Japanese Patent Publication No. 61-59914 has to apply heat having a high temperature to ink, so that the ink scorches and sticks to the head.
As an inkjet head which eliminates both of such drawbacks, there is the one using a piezoelectric element having a piezoelectric strain coefficient d.sub.33 (referred to as "d.sub.33 mode type" hereinafter). g
The d.sub.33 mode type ink-jet head comprises in its schematic structure a strip of piezoelectric material (piezoelectric element) and electrodes respectively formed on both of the confronting surfaces of the piezoelectric element, wherein the piezoelectric element is polarized into the same direction as that of an electric field which is formed across the electrodes to have the piezoelectric distortion constant d.sub.33. The piezoelectric element is extended and contracted in the direction of d.sub.33 by the electric field applied between the electrodes to apply pressure to an ink pressurizing chamber.
As the d.sub.33 mode type ink-jet head, there are already known the separate-liquid-chamber-type head as disclosed in Japanese Patent Publication No. 4-52213 and the extendible-liquid-chamber-type head as disclosed in Japanese Patent Publication No. 4-48622.
Of the d.sub.33 mode type ink-jet heads, a structure of the separate-liquid-chamber-type is shown in FIG. 19.
That is, a plurality of pressurizing chambers 202 formed by covering an upper plate 201 made of polysulfone, on the surface of which a plurality of grooves for ink flow path are formed, with a thin diaphragm 203 made of polysulfone. A plurality of electrode patterns are formed on the diaphragm 203.
On the other hand, a plurality of electrodes 206 are provided on a piezoelectric element 204 which is divided by slits 207. The piezoelectric element 204 is arranged adjacent to the pressurizing chambers 202 such that the electrodes 206 are connected to their corresponding electrode patterns 208 on the diaphragm 203.
An electrode 205 is formed on the surface of the piezoelectric element 204 on the opposite side to the electrodes 206. A U-shaped rigid material 209 forming a common electrode 210 is laminated on a surface forming the electrode 205. Further, the rigid material 209 is connected to the edge portions of the upper plate 201 where ink flow paths are not formed through the diaphragm 203.
Each of the electrode patterns 208 formed on the diaphragm 203 is electrically connected to each of the electrodes 206 provided at an end of the piezoelectric element 204, while the common electrode 210 is electrically connected to the electrode 205 provided on the other end of the piezoelectric element 204.
When a voltage is externally applied between each of the electrode patterns 208 formed on the diaphragm 203 and the common electrode 210 so as to generate an electric field in the same direction as that of polarization of the piezoelectric element 204, the piezoelectric element 204 divided by the slits 207 extends toward the direction of the electric field.
Hereupon, if the rigid material 209 and the upper plate 201 are firmly connected to each other and the rigid material 209 is made rigid enough to bear the stress of the piezoelectric element 204, the piezoelectric element 204 deflects the diaphragm 203 to pressurize ink for filling the pressurizing chambers 202. As the result, it is possible to eject ink via nozzles 211.
FIG. 20 shows a d.sub.33 mode type ink-jet head according to the other example of a prior art.
The ink-jet head shown in this drawing comprises a laminated piezoelectric actuator 214 formed by alternately laminating plate-shaped piezoelectric material 212 and internal electrodes 213 made of conductive material, instead of the piezoelectric element 204 in the ink-jet head shown in FIG. 19.
This structure in which the deformation of the plate-shaped piezoelectric material 212 is multiplied by the number of laminations can obtain a deformation in the thickness direction (d.sub.33 direction) large enough to eject ink droplets for the laminated piezoelectric actuator 214.
At this time, the laminated piezoelectric actuator 214 is also deformed in the direction perpendicular to that of polarization (d.sub.31 direction). However, the deformation in the d.sub.33 direction in which the amount of deformation is summed up by the number of laminations can generate higher pressure in the pressurizing chambers 202.
FIG. 21 shows the structure of an extendible-liquid-chamber-type d.sub.33 mode type ink-jet head.
It is composed of piezoelectric elements 222 each being a piezoelectric material strip such as PZT (Lead-Zirco-Titanate), the piezoelectric elements 222 being arranged in parallel between a conductive supporting plate 221 and an insulating cover plate 223 and fixed thereto.
A plurality of narrow channels are formed between the piezoelectric elements 222. These channels are composed of ink flow channels 225 serving as ink chambers/paths and dummy channels 226 serving as spacers, the ink flow channels 225 and dummy channels 226 being arranged alternatively.
The ink flow channels 225 are connected to a common ink chamber 227 which supplies ink to the ink flow channels 225 at an end thereof. The open ends of the ink flow channels 225 serve as printing nozzles.
The piezoelectric elements 222 is polarized into the direction perpendicular to the supporting plate 221 as indicated with an arrow 230, and electrodes 224 are provided corresponding to the ink flow channels 225 on the upper surface of the piezoelectric elements 222 on the side of the cover plate 223. Each of the electrodes 224 is provided for each pair of piezoelectric elements.
When a voltage is applied between the electrode 224 and the conductive supporting plate 221, the piezoelectric elements 222 arranged on both sides of an ink flow channel 225 extend toward the direction of thickness and contract toward the direction of width. As the result of deformation, the capacity of the ink flow channel 225 is increased.
When applying voltage to the electrode 224 is stopped, the two piezoelectric elements 222 return to their original shape, abruptly reducing the capacity of ink flow path. As the result, an ink droplet 228 is ejected from a printing nozzle formed at the end portion of the path.
A purpose of the present invention is to solve the following problems inherent in the ink-jet head having a structure in which strips of piezoelectric material are polarized into the direction of electric field to have the piezoelectric strain coefficient d.sub.33 as described above, i.e., the d.sub.33 mode type ink-jet head.
That is, a first problem is that the d.sub.33 mode type ink-jet head is structurally difficult to be miniaturized by increasing the density of arranging the printing nozzles and the degree of integrating the same.
For example, in case of the separate-liquid-chamber-type ink-jet head shown in FIG. 19, the piezoelectric elements 204 are arranged in a line forward the slits 207 among them for separating adjacent piezoelectric elements 204, so that the manufacturing limit of the slits 207 determines the pitch of the printing nozzles 211, and consequently it is impossible to densify the nozzle pitch. Incidentally, the limit of manufacturing slits using the wire-saw electron discharge method is up to about 150 to 200 slits per inch.
Moreover, an extendible-liquid-chamber-type ink-jet head shown in FIG. 21 also has a limited pitch of arranging the piezoelectric elements 222 same as the separate-liquid-chamber-type head and requires also the dummy channels 226 which do not eject ink, in every other row, so that it is all the more difficult to arrange nozzles with high density.
A second problem is that it is difficult to electrically connect power source to the piezoelectric elements to drive the same, and that the number of manufacturing steps is increased and the reliability of electrical connection is low.
For example, in case of the separate-liquid-chamber-type as shown in FIG. 19, the electrodes 205 and 206 have to be manufactured separately on the surface side of the piezoelectric element 204 confronting the diaphragm 203 and on the surface side thereof confronting the rigid material 209.
Furthermore, driving the piezoelectric elements 204 by these electrodes 205 and 206 alone requires tight junction leaving no space therebetween, which was very difficult in the machining technology.
Furthermore, there was a drawback that, in case of forming the other electrode on the piezoelectric element 204, the manufacturing cost was increased.
Furthermore, when the external signal lines are connected to the electrodes, troublesome work is required to connect them individually since the electrode patterns 208 on the diaphragm 203 and the common electrode 210 are differently positioned.
Furthermore, the electrode patterns 208 have to be previously made on the diaphragm 203 in advance, and also the material of the diaphragm 203 is limited to nonconductive one.
Also in case of the extendible-liquid-chamber-type one as shown in FIG. 21, the external signal lines have to be electrically and separatedly connected to the electrodes 224 and the conductive supporting plate 221 which serves as the common electrode.
A third problem is that nozzle holes for ejecting ink droplets are liable to be blocked with or leak ink.
That is, since the nozzles for ejecting ink are formed at the end portions of piezoelectric elements arranged with high density in both of the separate-liquid-chamber-type and the extendible-liquid-chamber-type, it is impossible to secure a space for installing a cap mechanism for preventing the evaporation of moisture from menisci, i.e., the liquid levels of ink in the nozzle holes, or a suction mechanism used when the nozzle holes are blocked with ink.
Moreover, even if the ink-jet head comprises a nozzle plate having a relatively large surface area and the nozzle holes formed at the end portions of piezoelectric elements, it is difficult to seal the nozzle plate so as to prevent ink from leaking, since very thin members such as the base plate, piezoelectric elements, diaphragm etc. have to be connected to the nozzle plate.
Also in case of employing a metal nozzle plate, since the upper plate 201 made of polysulfone and the diaphragm 203 have different coefficients of linear expansion, the change in temperature causes the deformation of members, resulting in the breakage of structure.
A fourth problem is that the energy loss or interference between pressurizing chambers is liable to occur, which causes the insufficiency or fluctuation of the ink ejecting force to reduce the performance of the ink-jet head.
When the piezoelectric element is displaced forward the direction of thickness (d.sub.33 direction), the displacement occurs toward the direction (d.sub.31 direction) perpendicular thereto.
Since the piezoelectric element and the diaphragm or the base plate are connected to each other through an electrode, the d.sub.31 direction displacement of the piezoelectric element causes unimorph deformation between the diaphragm and itself.
Accordingly, in case of the separate-liquid-chamber-type ink-jet head, the unimorph deformation causes the deflection of the diaphragm 203. As the result, there occurs a loss in the thickness-direction displacement of the piezoelectric element 204, so that extra energy is needed for ejecting ink.
In case of the extendible-liquid-chamber-type ink-jet head, the unimorph deformation deflects the supporting plate 221 and the insulating cover plate 223 to cause interference between ink flow channels.
A fifth problem is that it requires the high accuracy assembling and is difficult to manufacture.
In case of the separate-liquid-chamber-type ink-jet head as shown in FIG. 19, the piezoelectric element 204 and the rigid material 209 have to be connected to the diaphragm 203 with high accuracy of positioning which allows no deviation in order to transmit the minute deformation of the piezoelectric element 204 to the diaphragm 203.
In case of the conventional structure in which electrodes 206 are formed at an end of the piezoelectric element 204 and brought into contact with the diaphragm 203, however, it is impossible to flatten the portions of the rigid material 209 and the piezoelectric element 204 which are connected to the diaphragm 203 by machining such as surface grinding. Accordingly, it was difficult to connect with high accuracy.
An object of the present invention is to solve such problems in the d.sub.33 mode type ink-jet head as described above and provides an ink-jet head which causes little energy loss, and can be efficiently driven and manufactured at low cost due to its simple and small-sized structure, and has high reliability, and density, as well as the methods of manufacturing and driving such an ink-jet head.